In a cellular mobile communication system, handover between cells enables mobile communications across whole service areas. The handover may be also referred to as cell reselection. In some cases, cell transition during communication may be referred to as handover while cell transition during standby may be referred to as cell reselection, but these different types of transition are referred to as cell reselection without distinction herein. Theoretically, a time point or a location point when/where a connecting or waiting cell for a user apparatus should be switched corresponds to a point when/where a magnitude relationship is reversed between reception signal strength of signals transmitted from a source base station (source eNB) and reception signal strength of signals transmitted from a target base station (target eNB). Meanwhile, the cell reselection imposes a high signal processing workload on both a system and a user apparatus and accordingly should not be conducted excessively. For example, in the handover during communication, processing workload may arise such as path switching or context transferring between network nodes for user apparatuses. Also, in the cell reselection during standby, processing workload on a user apparatus for receiving broadcast information at a target cell may arise, which may influence battery energy consumption. For these reasons, the cell reselection is commonly initiated in consideration of some hysteresis.
FIG. 1 illustrates an exemplary correlation of reception signal power and hysteresis. In the illustration, the cell reselection is initiated after a state of the reception signal strength of signals transmitted from a target eNB being higher than the reception signal strength of signals transmitted from a source eNB by greater than or equal to a hysteresis threshold Qhyst has been maintained for a time period longer than or equal to a “time to trigger” T (second). The hysteresis threshold and the time to trigger are transmitted to a user apparatus in broadcast information BCH. In this manner, unnecessary cell reselection can be reduced near cell boundaries. Thus, it is preferred that the time to trigger be made longer to perform only truly necessary cell reselection.
Meanwhile, cells are variously arranged and users are traveling in various directions in a mobile communication system. For example, Case 1 in FIG. 2 schematically illustrates that a user is passing through several cells having a relatively large size at a certain velocity v. Case 2 schematically illustrates that a user is passing through several cells having a relatively small size at the same velocity v. In Case 2, since the cell reselection should be performed frequently, it is preferred that the time to trigger be set to be shorter. In Case 1, on the other hand, the cell reselection does not have to be performed so frequently. Rather, if the time to trigger is made shorter in Case 1, there is a risk that unnecessary cell reselections may be performed. In conjunction with this point, the time to trigger Treselection is set to be shorter in a UMTS (Universal Mobile Telecommunication System) if the number of cell reselections performed previously by a user apparatus is greater than a predefined value. Adjustment of Treselection in UMTSs is disclosed in 3GPP TS25.331 V7.5.0 and 3GPP TS25.304 V7.2.0, for example. Particularly, parameters are disclosed in TS25.331, 10.3.2.3.